[Lowfer] R75 Frequency Accuracy

[Douglas D. Williams]

The main shortcomings of the R-75 for direct VLF/LF receive occur
mostly below 200 kHz, where there are numerous spurs that can be heard with
nothing but a shielded 50 ohm "dummy load" connected to the antenna input
jack, and as noted in two separate Lowdown reviews of two different R-75
receivers. The further below 200 kHz you go, the more numerous the spurs.
The R-75 can only directly tune down to 30 kHz. A good quality upconverter
eliminates almost all of the spurs, and allows the reception of down to
around 5 kHz, but also introduces additional frequency drift.

I agree that the R-75 is very stable, especially with the TCXO option.

73, Doug KB4OER




On Wed, Jan 23, 2013 at 8:34 PM, <jrusgrove at comcast.net> wrote:

> Same here. Have two R-75s that are used almost daily for LF/MF activities.
> Both have the TCXO option and remain within a tenth of the indicated
> frequency. Agree with John ... the shortcomings of the R-75 have been
> overstated. Both are used to receive LF and MF directly without
> upconversion. Performance in this application is on par with the the Harris
> RF-590 - the main LF/MF receiver - and other 'premium' receivers in use
> here.
>
> Jay W1VD  WD2XNS  WE2XGR/2
>
>
> ----- Original Message ----- From: "John Andrews" <w1tag at charter.net>
> To: "Discussion of the Lowfer (US, European, &amp;UK) and MedFer bands" <
> lowfer at mailman.qth.net>
> Sent: Wednesday, January 23, 2013 7:59 PM
> Subject: Re: [Lowfer] R75 Frequency Accuracy
>
>
>
>  Pat,
>>
>> Like Dex, I keep the R75 on all the time, so the warm-up period is not a
>> factor. I don't use an upconverter, so I am receiving directly through the
>> R75. The R75's shortcomings have been overstated, but I really don't want
>> to get into an argument over it. It has worked out very well for me in all
>> applications.
>>
>> Regarding calibration, it would be worth the time to take the top cover
>> off, and adjust L2 in the PLL unit. I would recommend doing that up at 10
>> MHz, since you now have a good source. You can do this with Argo, assuming
>> that you have done the sound card calibration first so that the only error
>> would be in receiver.
>>
>> At LF, my R75s are both within a couple of tenths of a Hz of being on
>> frequency.
>>
>> John, W1TAG
>>
>> On 1/23/2013 6:37 PM, pbunn at patbunn.com wrote:
>>
>>>
>>> I have an older R75 that I bought 15 years ago and rarely used until
>>> recently.
>>>
>>> I use it for LF work with a home brew RX up-converter and it has worked
>>> well but frequency accuracy has been quite a bit off. I attributed this
>>> to my up converter being a bit off and combined with the R75 error
>>> combining to cause the problem.
>>>
>>> Recently I have added a Trimble Thunderbolt and have begun to find out
>>> where the error is. My up-converter is now phase locked to the GPS 10
>>> Mhz output and yet my receiver is still 8 Hz off .
>>>
>>> Is this more than might be expected from a typical R75 or normal. It
>>> seems somewhat excessive to me. I plan to add W1VDs circuit to add GPS
>>> control to the R75 but want to make sure that the R75 is not excessively
>>> out of spec.
>>>
>>> The sound card was calibrated with a GPS disiplined 1KHz signal and
>>> found to be within .1Hz using ARGO before calibration so the error is
>>> not there.
>>>
>>> Any comments would be appreciated.
>>>
>>> Pat
>>> N4LTA
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